Thermoplastics, such as polyethylene terephthalate (PET), used in injection molding processes are often supplied as beads or grains of resin. The beads are fed into heating units to liquefy the plastic before injection into the mold. The resin, particularly for highly hygroscopic plastics, is generally dried to remove excess moisture before heating and injection, as elevated moisture levels can lead to injection-molded products that are structurally flawed, aesthetically flawed, or both.
Resin drying machines conventionally include dehumidifying mechanisms to dry moisture-laden air that has passed through the resin before recycling that air to remove further moisture from the resin. Two dominant dehumidifying technologies in use with resin dryers are dual-bed dehumidifiers, and rotary dehumidifiers.
Dual-bed dehumidifiers (also referred to as twin tower dehumidifiers) generally consist of a pair of towers, each containing a bed of desiccant material such as a molecular sieve. Humid process air is blown through a first one of the towers, where it is dried by the desiccant material, before being routed to a resin drying chamber to remove moisture from resin beads in the chamber. As the bed of desiccant becomes saturated with moisture from the process air, its effectiveness in absorbing further moisture falls over time. The first tower is then switched to a regeneration mode, and process air is redirected through the second tower instead of the first tower. In the regeneration mode, heated air is passed through the bed of desiccant in the first tower to remove accumulated moisture. Although dual-bed dehumidifiers can achieve low dew points in process air output, the humidity of the outgoing process air can suffer from inconsistency. For example, there may be sharp changes in the humidity of the process air when one bed approaches saturation and process air is switched to the other bed. Further, maintaining dual-bed dehumidifiers can be laborious and time-consuming; for example, inspecting or replacing the desiccant in a tower requires that the tower be disassembled and cleaned out by hand.
Rotary dehumidifiers employ a disk containing a desiccant material, generally a silica gel containing a suspension of molecular sieve material. The disk rotates continuously, and process air is blown through one part of the disk while heated regeneration air is blown through another part of the disk. The continuous handling of process and regeneration air can provide for more consistent performance characteristics than can be obtained from dual-bed systems. However, rotary dehumidifiers generally do not achieve the same low dew points as dual-bed dehumidifiers unless the process air entering the disk is sufficiently cool. In some rotary machines, the process air is therefore cooled before passing through the disk, and then must be heated before travelling to the resin dryer. The large changes in temperature consume significant amounts of energy, reducing the efficiency of rotary dehumidifiers. Further, the seals between process and regeneration air conduits and the disk wear out over time. Until the seals are replaced, the efficiency of the dehumidifier may be further reduced.